Examples of electron donor-acceptor dyads reported conventionally include a number of dye molecules such as porphyrin, and the charge-separated states thereof have been reported. From the viewpoint of industrial applicability, these electron donor-acceptor dyads are required to have properties such as longevity, high oxidizing power, and high reducing power for the charge-separated state. In order further to improve the properties, further studies have been made.
However, from the viewpoint of industrial applicability, no electron donor-acceptor dyad with sufficient reducing power has been reported yet. Therefore, among the aforementioned respective properties of the electron donor-acceptor dyads, improvement of the reducing power is an important study subject. For instance, examples of the electron donor-acceptor dyad that provides a charge-separated state having the longest longevity so far include a 9-mesityl-10-methylacridinium ion (see Nonpatent Documents 1 and 2). However, the electron-transfer state (charge-separated state) has low energy and therefore the reducing power thereof is not so strong, which has been a problem.    [Nonpatent Document 1] S. Fukuzumi, H. Kotani, K. Ohkubo, S. Ogo, N. V. Tkachenko, H. Lemmetyinen, J. Am. Chem. Soc., 2004, 126, 1600    [Nonpatent Document 2] K. Ohkubo, H. Kotani, S. Fukuzumi, Chem. Commun. 2005, 4520.